What is nagamani does it exist?

Nagamani is a myth. Many people have fallen prey to such fake claims. It is said to be a "cobra pearl." See the link listed below:

Just like other myths that Aliens exists, Jesus was reborn on Good Friday, Time travel is possible and Universe has many layers. But these beliefs are supported by scientific THEORIES or ancient texts. Because their existence cannot be supported by verifiable proofs that are 100% facts. So its up to individuals if they want to follow their faith or deny the existence which they cannot see.